Encontrei o problema: o driver da Nvidia não funciona com os dois monitores.
Ontem recebi um novo monitor que conectei com HDMI ao meu laptop. Tudo funcionou bem. Quando eu conecto meu cabo HDMI hoje, a tela do laptop desliga e exibe a foto em anexo a cada 2-3 segundos, que também é exibida a cada 2-3 segundos no monitor. Conectar o cabo HDMI não resolve o problema da tela do laptop.
EDITAR:
lspci -vvv|grep -A 11 'VGA '
00:02.0 VGA compatible controller: Intel Corporation 3rd Gen Core processor Graphics Controller (rev 09) (prog-if 00 [VGA controller])
Subsystem: Acer Incorporated [ALI] Device 0647
Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx+
Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0
Interrupt: pin A routed to IRQ 28
Region 0: Memory at b3000000 (64-bit, non-prefetchable) [size=4M]
Region 2: Memory at c0000000 (64-bit, prefetchable) [size=256M]
Region 4: I/O ports at 3000 [size=64]
Expansion ROM at <unassigned> [disabled]
Capabilities: <access denied>
Kernel driver in use: i915
--
01:00.0 VGA compatible controller: NVIDIA Corporation GF117M [GeForce 610M/710M/820M / GT 620M/625M/630M/720M] (rev a1) (prog-if 00 [VGA controller])
Subsystem: Acer Incorporated [ALI] GeForce 710M
Control: I/O+ Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
Latency: 0
Interrupt: pin A routed to IRQ 35
Region 0: Memory at b2000000 (32-bit, non-prefetchable) [size=16M]
Region 1: Memory at a0000000 (64-bit, prefetchable) [size=256M]
Region 3: Memory at b0000000 (64-bit, prefetchable) [size=32M]
Region 5: I/O ports at 2000 [size=128]
Capabilities: <access denied>
Kernel driver in use: nvidia
Encontrei o problema: o driver da Nvidia não funciona com os dois monitores.